Set fx.StopTimeout inside the container stop grace (closes #134)

fx defaults to a 15s stop timeout and the Dockerfile sets no grace
override, so Docker SIGKILLed at 10s and the bounded shutdown #130 built
— including the log line that tells an operator a component is wedged —
was unreachable in the image this repo produces.

Sets fx.StopTimeout to 5s, and lowers the HTTP drain to 3s so a
full-length drain no longer exhausts the whole sequence budget and skip
every later hook, database close included. The Sentry flush, which runs
in the same hook and honours no context, is clamped to the remaining
stop budget less a 2s tail reserve, so a stalled flush drops Sentry
events rather than the database close.

Also fixes a latent coin flip in the shared stop-hook waiter, which
reported "shutdown timed out" about half the time for a component that
drained cleanly against an already-expired context.

Independently reviewed three times. The final reviewer derived a
stronger invariant than the implementation claims — the server hook's
absolute end is bounded at stopTimeout minus the reserve regardless of
drain length or of time consumed by preceding hooks — and confirmed the
guard's 10ms sweep cannot step over the maximum, since both breakpoints
land on its grid. Both Sentry probe arms, the docker stop demo and every
mutation were reproduced independently.

Known residual, filed separately: the HTTP drain itself is not clamped
by the reserve, so slow preceding hooks can still jointly exhaust the
budget. Demonstrated with a 2.2s sweeper delay.

internal/lifecycle/lifecycle.go

@@ -38,6 +38,29 @@ func WaitForShutdown(
 		wg.Wait()
 	}()
 
+	return waitDone(ctx, log, component, done)
+}
+
+// waitDone waits for done to close, bounded by ctx.
+//
+// The non-blocking preamble is load-bearing. When the component has
+// already drained and ctx has already expired, both cases of the
+// bounded select are ready and Go picks between them uniformly at
+// random, so a clean shutdown would be reported as a timeout about
+// half the time. Draining wins: the goroutines are gone, and there
+// is nothing left for the operator to act on.
+func waitDone(
+	ctx context.Context,
+	log *slog.Logger,
+	component string,
+	done <-chan struct{},
+) error {
+	select {
+	case <-done:
+		return nil
+	default:
+	}
+
 	select {
 	case <-done:
 		return nil